GIMP
GIMP is the GNU Image Manipulation Program used for photo retouching, image composition and image authoring. More information and the latest  version available at:
/www.gimp.org/

Image J
MacBiophotonics Image J is an advanced image processing program with a large number of native functions supplemented by an ever increasing number of “plugins”. More information and the latest version available at:
/www.macbiophotonics.ca/imagej/

Combine Z
This software combines pictures to increase depth of focus. More information and the latest  version available at:
/www.hadleyweb.pwp.blueyonder.co.uk/CZ5/combinez5.htm

ImageTool
An advanced image processing and analysis program with various functions including dimensional measurements and automatic or manual object counting. More information and the latest version available at:
/ddsdx.uthscsa.edu/dig/itdesc.html